1. Field of the Invention
The invention relates to a polarizer for transforming a light beam incident along an optical axis into an exiting light beam that has a prescribed distribution of locally varying polarization states over its cross-section.
2. Description of Related Art
In the case of microlithographic projection illumination systems, the polarization state of the light employed for imaging is frequently selectively controlled in order to provide higher imaging fidelity. Polarizers similar to those described here located in the illumination system and/or the projection lens may be employed for this purpose.
A polarizer of said type that is intended for use on an illumination system and generates an exiting light beam that is polarized largely in the radial direction over its entire cross-section is known from German Pat. No. DE 195 35 392. Radial polarization is well-suited for use with lenses having typical numerical apertures ranging from about 0.5 to about 0.7 on their image sides and photoresists that have no antireflection coatings, where it is used in order to suppress disturbances due to polarization-sensitive reflections at said photoresists that occur at angles of incidence close to the Brewster angle, which yields optimal coupling of light into the photoresist.
One embodiment of a polarizer for transforming incident linearly polarized light into radially polarized light consists of numerous, hexagonal, half-wave plates covering its entire surface, where said half-wave plates are fabricated from a birefringent material whose crystal axis is aligned with respect to the direction of the incident light beam such that each half-wave plate deflects the direction of polarization of incident light along a radius vector that is aligned on said optical axis and intersects said half-wave plate. This segmented arrangement generates the desired, prescribed distribution of locally varying, preferred, polarization states exclusively by rotating the polarization state of incident light, rather than by filtering, which allows attaining high transmittances. Fabricating polarizers of said type is difficult due to their segmented construction. Furthermore, the polarizer, whose thickness is determined by the wavelength employed and the difference in refractive indices orthogonal to the crystal axis for mutually orthogonally polarized field components, may be very thin, depending upon the wavelength involved and the material employed, which will make mounting elements of said type more difficult.
A projection illumination system for use in microlithography that also introduces radially plane-polarized light in order to prevent distorted imaging on the photoresist is known from U.S. Pat. No. 5,365,371 and the associated CIP-Patent, U.S. Pat. No. 5,436,761, which illustrate several embodiments of polarization filters that lead to high transmission losses. Radial polarizers are installed in the vicinity of the plane of the projection lens' pupil or system stop.
A catadioptric projection lens that may be configured such that it has a segmented polarizing plate situated in the plane of its pupil is known from U.S. Pat. No. 5,691,802. Said segmented plate has a circular inner zone and an annular outer zone that generate mutually orthogonal, linearly polarized light and have differing refractive indices, thereby generating a pair of noninterfering light beams that create mutually distinct image planes, which collectively serves to increase the lens' depth of field.
Polarizing axicon arrangements that, when used in combination with conical surfaces, may be used for generating exiting light beams that are preferentially either radially or tangentially polarized with respect to their optical axis, i.e., have an axi-symmetric distribution of varying, preferred, polarization directions, at every point on their cross-sections, are known from U.S. Pat. No. 4,755,027.